I tend not to take any money with me to the beach. If it can't be avoided, then I take the smallest amount possible and tuck it into the pocket of my shorts (NOT in a wallet which is too obvious).
If I'm feeling particularly vulnerable, I have tucked it into a small waterproof bag (one that I would normally use for my camera to take photos while snorkelling).
I think they are also smaller waterproof plastic cases that you can get to carry around your neck to hold a room key and a small amount of cash. Haven't tried one though.
I would always leave my main wallet back in the room, locked up in my pack.